Method and apparatus for monitoring film thickness between rotatable rolls

Abstract

An improved method and apparatus for monitoring the thickness of a film includes a sensor roll which cooperates with a compliant roll to form a nip through which the film passes. As the two rolls rotate relative to each other, magnets disposed in the sensor roll are sequentially moved to and from the nip. As a magnet moves by the nip, the field emanating from the magnet is concentrated by a body of material having a relatively high magnetic permeability and disposed in the compliant roll. This results in an induced voltage being generated in a sensor coil. The output from the sensor coil is transmitted to control circuitry which effects operation of a suitable apparatus to control the thickness of the film.

Having described specific preferred embodiments of the invention, the following is claimed: 
     
       1. An apparatus for monitoring the thickness of a film of liquid, said apparatus comprising a first roll which is rotatable about a first axis, said first roll including material having a relatively high magnetic permeability, a second roll which is rotatable about a second axis, said second roll cooperating with said first roll to form a nip through which the film of liquid passes, means for rotating said first roll about the first axis and said second roll about the second axis, a plurality of magnets mounted in said second roll at locations which are spaced apart along the second axis so that rotation of said second roll moves said magnets toward and away from the nip along circular paths which are spaced apart along the width of the film, said magnets being spaced apart about the circumference of said second roll whereby rotation of said second roll moves each of said magnets in turn to a location on its circular path of movement in which it is closest to the nip, and a plurality of sensor means, each of said magnets having an associated sensor means for sensing the flux density of its magnetic field provided upon movement of the associated magnet to the location on its path of movement which is closest to the nip and which flux density is representative of the thickness of the film of liquid, and to generate a signal representative of film thickness. 
     
     
       2. An apparatus as set forth in claim 1 wherein said material having a relatively high magnetic permeability includes a plurality of sections having cylindrical outer side surfaces which engage the film of liquid and which cooperate with said second roll to form the nip through which the film of liquid passes, said first roll including means for enabling each of said sections to be radially offset relative to adjacent sections under the influence of fluid pressure forces applied to said first roll by the film of liquid. 
     
     
       3. An apparatus as set forth in claim 2 further including liquid film thickness control means for varying the thickness of a longitudinally extending area of the film which engages one of the sections of said first roll. 
     
     
       4. An apparatus as set forth in claim 3 wherein the path of movement of each of said magnets is disposed radially outwardly of a different one of the sections of said first roll. 
     
     
       5. An apparatus as set forth in claim 1 wherein each of said magnets includes a core member connected with said second roll for rotation therewith about the second axis and an excitation coil extending around said core member, said apparatus further including means for connecting each of said excitation coils with a source of direct current to maintain the magnetomotive force of each of said magnets substantially constant during rotation of said second roll. 
     
     
       6. An apparatus as set forth in claim 5 wherein said sensor means includes a plurality of sensor coils each of which extends around one of said core members. 
     
     
       7. The apparatus of claim 5 wherein the excitation coils of the magnets are electrically connected in series across said source of direct current. 
     
     
       8. The apparatus of claim 7 wherein the sensor means are electrically connected in series, said apparatus further including a demultiplexing means connected to the series connected sensor means for separately generating a signal indicative of the flux density monitored by each sensor means. 
     
     
       9. The apparatus of claim 8 further including liquid film thickness control means responsive to the signals generated by said demultiplexing means for varying the thickness of the liquid film in an area corresponding to the area being monitored by the demultiplexing means. 
     
     
       10. An apparatus for monitoring the thickness of a film of ink, said apparatus comprising: a first member formed of material having a relatively high magnetic permeability and disposed on a first side of the film;   magnetic means for providing a magnetic field having a substantially constant magnetomotive force and having a flux density, said magnetic means disposed on a second side of the film;   means for effecting relative movement between said magnetic means and said first member such that said magnetic means and said first member approach to a distance which is a function of the film thickness, said magnetic field interacting with said first member at said distance such that said flux density varies in accordance with said distance; and   sensor means for sensing the flux density of said magnetic field at said distance and for generating a signal indicative thereof;   wherein said first member is a first roller which is rotatable about a first axis and which has a first circular outer surface engageable with a first side of the film of ink, said apparatus further including a second roller which is rotatable about a second axis and which has a second circular outer surface engageable with a second side of the film of ink, said magnet means being disposed within said second roller and being connected with said second roller for movement with said second roller, said means for effecting relative movement being operable to (i) move at least one of said rollers away from the other of said rollers under the influence of fluid pressure forces applied against said one roller by the film of ink upon an increase in the thickness of the film of ink and (ii) rotate said second roller about the second axis to move said magnet means towards said first roller to said distance.   
     
     
       11. An apparatus for monitoring the thickness of a film of ink, said apparatus comprising: a first member formed of a material having a relatively high magnetic permeability and disposed on a first side of the film;   magnetic means for providing a magnetic field having a substantially constant magnetomotive force and having a flux density, said magnetic means disposed on a second side of the film;   means for effecting relative movement between said magnetic means and said first member such that said magnetic means and said first member approach to a distance which is a function of the film thickness, said magnetic field interacting with said first member at said distance such that said flux density varies in accordance with said distance; and   sensor means for sensing the flux density of said magnetic field at said distance and for generating a signal indicative thereof;   wherein said first member is a first roll which is rotatable about a first axis and has first surface means for engaging a first side of the film, said apparatus further including a second roll which is rotatable about a second axis which is parallel to said first axis and which has second surface means for engaging a second side of the film, said first and second surface means cooperating to define a nip through which the film passes, said magnet means including a plurality of magnet members carried by said second roll and disposed adjacent said second surface means in a helical array about the second axis, said means for effecting relative movement being operable to rotate said second roll about said second axis to move each of said magnet members in turn to and from said distance.   
     
     
       12. A method of monitoring the thickness of a film of liquid, said method comprising the steps of moving the film of liquid through a nip formed between first and second rolls by rotating the first and second rolls about their central axes, providing a magnet carried by said second roll, said magnet producing a magnetic field having a substantially constant magnetomotive force and having a flux density, providing a relatively high magnetically permeable material carried by said first roll, rotating said first roll and said second roll to move the magnet and said magnetically permeable material to a distance that is a function of the film thickness, said flux density varying as a function of said distance, and sensing the flux density of said magnetic field at said distance and generating a signal representative thereof, the signal generated when said magnet and said magnetically permeable material are at said distance being indicative of the film thickness. 
     
     
       13. A method as set forth in claim 12 further including the step of varying the thickness of the liquid film as a function of the flux density to maintain a desired liquid film thickness. 
     
     
       14. An apparatus for monitoring the thickness of a film of liquid, said apparatus comprising first and second rolls having outer surfaces which cooperate to form a nip, means for rotating the first and second rolls about their central axes to move the film of liquid through the nip, a relatively high, magnetically permeable material carried by said first roll, a magnet carried by said second roll for rotation therewith about the central axis of said second roll for providing a magnetic field having a substantially constant magnetomotive force and having a flux density, said flux density varying as a function of the distance between said magnet and the relatively high, magnetically permeable material, sensor means associated with said magnet for sensing the flux density of said magnetic field and generating a signal indicative thereof, said magnet being movable by rotation of said second roll in a direction toward the nip thereby increasing the flux density, said magnet being movable by rotation of said second roll in a direction away from the nip thereby decreasing the flux density, the maximum flux density being indicative of the thickness of the film of liquid. 
     
     
       15. An apparatus as set forth in claim 14 wherein said magnet includes a core member and an excitation coil extending around said core member, said apparatus further including circuit means for connecting said excitation coil with a source of direct current to maintain a constant magnitude flow of current through said excitation coil during rotation of said first roll. 
     
     
       16. An apparatus as set forth in claim 14 further including means for varying the thickness of the liquid film as a function of the flux density to maintain a desired liquid film thickness.